The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a bi-zone water transport plate that may be used within a fuel cell for transporting reactant, product and coolant fluids to, through and from the fuel cell, for conducting electricity from one cell to an adjacent cell, for providing a barrier to transfer of gaseous reactants between adjacent cells, and/or for providing mechanical integrity to the fuel cell.
Fuel cells are well-known and are commonly used to produce electrical energy from reducing and oxidizing reactants fluids to power electrical apparatus such as apparatus on-board space vehicles, or on-site generators for buildings. A plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids as part of a fuel cell power plant. Each individual fuel cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
It is known to utilize one component of a fuel cell to assist in the accomplishment of a variety of water management and related tasks. Such a component is typically formed of a porous carbon body and is commonly referred to under various names including xe2x80x9ccooler platexe2x80x9d, xe2x80x9cwater transport platexe2x80x9d, xe2x80x9cseparator platexe2x80x9d, xe2x80x9cbi-polar platexe2x80x9d, xe2x80x9cend platexe2x80x9d, among other names. For example, in U.S. Pat. No. 6,024,848 that issued on Feb. 15, 2000 to Dufner et al., which patent is owned by the assignee of all rights in the present invention and which patent is hereby incorporated herein by reference, a water transport plate is shown that defines a plurality of coolant water feed channels on a planar surface of the plate and on an opposed surface a network of reactant gas distribution channels is defined. Such a water transport plate is typically a porous carbon body and the plate must perform a variety of functions. It must transport water from coolant channels through the body to gaseous reactant channels to humidify a reactant fluid within the gas reactant channels; it must remove product water generated at the cathode electrode across the body into the coolant water channels to prevent flooding of the cathode electrode; it must form a gaseous barrier to prevent mixing of fuel and oxidant reactant fluids on opposed sides of the plate; it must conduct electricity or electrons from one cell to an adjacent fuel cell in a fuel cell stack assembly; it must conduct waste heat generated within the fuel cell to the coolant fluid; it may provide a distribution network for oxidant and reducing fluid reactants; and, it must provide mechanical support and integrity to the fuel cell.
Therefore, such a water transport plate must be porous, wettable to water, have a high rate of water permeability and have a high bubble pressure. However, characteristics of the water transport plate that are appropriate for a high bubble pressure, are inconsistent with characteristics appropriate for a high rate of water permeability. For example, to increase bubble pressure to thereby enhance a gaseous seal between gaseous oxidant and fuel reactants on opposed sides of the water transport plate, it is appropriate to have a small pore size of the pores within the plate. However, to enhance permeability of the body to coolant or product water, it is desirable to have a large pore size or pore diameter. Known fuel cell water transport plates have a generally uniform pore size, and the pore size is typically determined as a compromise between requirements for water permeability based upon plate thickness and an area of contact surfaces of the plate and requirements for bubble pressure based upon a maximum operational pressure differential between fluids passing within flow fields adjacent opposed contact surfaces of the plate.
Accordingly, there is a need for a water transport plate for a fuel cell that provides for increased bubble pressure and water permeability of the plate.
The invention is a bi-zone water transport plate for a fuel cell wherein the bi-zone water transport plate is secured in fluid communication with a catalyst of the fuel cell. The bi-zone water transport plate includes a water permeability zone and a bubble barrier zone, wherein the bubble barrier zone extends between all reactive perimeters of the plate, has a pore size of less than 20 microns, and has a thickness of less than 25 percent of a shortest distance between opposed contact surfaces of the plate, and wherein the water permeability zone has a pore size of at least 100 percent greater than the pore size of the bubble barrier zone, and has a thickness of greater than 75 percent of the shortest distance between the opposed contact surfaces of the plate. These thickness ratios are selected to maximize the water permeability through the bubble barrier zone while minimizing a water inventory of the water permeability zone.
In a preferred embodiment, the bubble barrier zone has a pore size of less than 5 microns, a thickness of between about 0.010-0.025 centimeters, and the water permeability zone has a pore size of between about 10-20 microns and a thickness of between about 0.075-0.200 centimeters. In an alternative embodiment, a cathode or an anode substrate is disposed between the bi-zone water transport plate and in a cathode or anode catalyst of the fuel cell, the pore size of the water permeability zone is about 40 percent of the pore size of the cathode or anode substrate, and the pore size of the bubble barrier zone is about 20 to 50 percent of the pore size of the water permeability zone.
Additionally, the bubble barrier zone may be defined on either of the opposed contact surfaces of the bi-zone water transport plate, or may be defined within the plate between portions of the water permeability zone. The bubble barrier zone may also overlie an entire perimeter edge of the bi-zone water transport plate to form an edge seal of the plate.
Accordingly, it is a general object of the present invention to provide a bi-zone water transport plate for a fuel cell that overcomes deficiencies of prior art water transport plates for fuel cells.
It is a more specific object to provide a bi-zone water transport plate for a fuel cell that enhances both bubble pressure and water permeability of the plate.
It is yet another object to provide a bi-zone water transport plate for a fuel cell that may provide gaseous fluid sealing at either of opposed contact surfaces of the plate or at a middle portion of the plate.
It is still a further object to provide a bi-zone water transport plate for a fuel cell that provides for edge sealing of the plate with a bubble barrier zone.
It is yet another object to provide a bi-zone water transport plate for a fuel cell that depresses a freezing temperature of water remaining within the bi-zone water transport plate.
These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.